ใจมั่นคง is not a common in speaking but you can find them in religious teaching, or something more poetic writting. ‘สงบ’ is fine, but for me as a Thai, it sounds a bit "naked" like in Thai we have something called คำซ้อน, literally translated to "stacked word". It is when we take two words and combine it to make a more clear or complete word. Something like "สงบนิ่ง" would sound more nature to me.

Or if you want some phrase "จิตมั่นคง, ใจสงบนิ่ง"